1. Field of the Invention
The present invention relates to an optical disk recording/reproducing apparatus for recording information on an optical disk and for reproducing information recorded on the optical disk, and more particularly, to an enhancement of a tracking servo of an optical pickup.
2. Description of the Related Art
In general, optical disk recording/reproducing apparatuses employ tracking servo schemes, that is, a control scheme for enabling a laser beam emitted from an optical pickup to accurately follow a track formed on an optical disk. A differential push-pull (DPP) scheme is one of the tracking servo schemes. In the DPP scheme, a tracking error signal is generated by calculating signals output from respective light-receiving elements for receiving one main beam and two sub beams.
Specifically, in the DPP scheme, refracting means is provided at a position in a path of the laser beam emitted from a laser source of the optical pickup, three spots are formed on the optical disk by three light beams of one zero-th order refracted light beam (main beam) and two first order refracted light beams (sub beams), and lights reflected therefrom are received by photo-detectors. The main spot formed by the main beam is used to record or read out a signal, and the sub spots formed by the sub beams are used to detect the tracking error signal.
A main photo-detector for receiving a main beam reflected from the main spot is horizontally and vertically partitioned into four elements. Each of sub photo-detectors for receiving two sub spots is horizontally partitioned into two elements. Output signals of the four partitioned elements of the main photo-detector are indicated by A, B, C, and D, respectively. Output signals of two partitioned elements of one of the sub photo-detectors are indicated by E and F, respectively, and output signals of two partitioned elements of the other of the sub photo-detectors are indicated by G and H (see FIG. 2).
A main push-pull signal (MPP signal) can be obtained from the output signals A, B, C, and D of the main photo-detector by using the following expression (1).MPP=(A+D)−(B+C)  (1)
One sub push-pull signal (SPP1 signal) can be obtained from the output signals E and F of the one sub photo-detector by using the following expression (2), and the other sub push-pull signal (SPP2 signal) can be obtained from the output signals G and H of the other sub photo-detector by using the following expression (3).SPP1=F−E  (2)SPP2=H−G  (3)
As a result, a sub push-pull signal (SPP signal) can be represented by the following expression (4).SPP=SPP1+SPP2=(F+H)−(E+G)  (4)
Accordingly, a DPP tracking error signal (DPP•TE signal) can be obtained by using the following expression (5).DPPTE=[(A+D)−(B+C)]−k*[(F+H)−(E+G)]  (5)
Here, k is a coefficient used to correct the level ratio between the main push-pull signal (MPP signal) and the sub push-pull signal (SPP signal). The coefficient k corresponds to the gain value of an amplifier for amplifying the sub push-pull signal (SPP signal).
Conventional optical disk recording/reproducing apparatuses are disclosed in JP-A-2003-317274, JP-A-2001-307352, JP-A-2002-183992, and in JP-A-2003-006893.